The objective of this proposed research is to determine wheher low levels of serum protease inhibitors or polypeptide inactivators may predispose certain individuals to experience anaphylaxis-like reactions to radiographic contrast media or to develop varieties of angioedema or chronic urticaria which currently are considered to be "idiopathic". The parameters to be studied are blood levels of hemolytic complement, complement components, carboxypeptidase B, alpha2-macroglobulin, alpha1-antitrypsin, antithrombin III, inter-alpha-inhibitor, C1 esterase inhibitor and anti-chymotrypsin activity. An effort will be made to obtain these measurements during acute reactions to radiographic contrast media as well as after recovery from the reaction. Also, release of histamine and the levels of these protease inhibitors will be measured after adding radiographic contrast media to blood in vitro, as well as complement components.